This invention pertains to selective latent catalysts showing high selectivity and reactivity in advancing liquid epoxy resins to high molecular weight, essentially unbranched epoxy resins which are solid at room temperature.
The instant process comprises reacting together, in the presence of a catalyst, an epoxy liquid resin which contains more than one epoxy group, preferably two, and a phenol containing at least two hydroxyl groups. The reaction proceeds as a step-growth polymerization process and has been termed a fusion or advancement process.
An important property of the catalyst in advancement processing is the selectivity conferred on the phenolic hydroxylepoxide reaction. Highly selective catalysis ensures the production of linear polyhydroxyethers and is the highly desired result. Poor selectivity allows the aliphatic hydroxyl group, generated upon opening of the oxirane ring, to react with an epoxy group: ##STR1## The resultant polymer in the latter case is branched or crosslinked and is deficient in chain flexibility.
Along with the forementioned selectivity, advancement catalysts should be sufficiently reactive to enable "linear" solid advanced epoxy resins of high molecular weight to be formed at economically attractive rates. High reaction rates are accompanied by large exotherms due to opening of the epoxy ring and the liberated heat serves to sustain the rapid attainment of a higher molecular weight product.
Many compounds have been suggested or described as catalysts for the advancement of liquid epoxy resins. However, some are not selective causing the aforementioned branching or crosslinking to occur while others are ineffective in promoting the highly exothermic advancement reactions at practical rates to obtain a satisfactorily advanced linear solid epoxy resin.
Phosphonium halide catalysts, such as methyl or ethyl triphenylphosphonium iodide or bromide, are described in U.S. Pat. Nos. 3,447,990 and 3,547,885.
Organic phosphine catalysts, such as triphenylphosphine or tributylphosphine, are disclosed in U.S. Pat. No. 3,547,881.
Phosphonium salts of carboxylic acids, such as ethyltriphenylphosphonium acetate or its acetic acid complex or phosphonium phosphate esters, are described in U.S. Pat. No. 3,948,855.
Phosphorus containing catalysts which are the reaction products of trihydrocarbyl phosphines with maleic anhydride and the hydrolyzed derivatives thereof are disclosed in U.S. Pat. Nos. 4,048,141 and 4,131,633.
Trisubstituted (2,5-dihydroxyphenyl)phosphonium hydroxide inner salts are described as catalysts in U.S. Pat. No. 4,132,706.
Alkylene phosphoranes of the general formula EQU R'.sub.3 P.dbd.CR"R"'
wherein each of the groups represented by R' may be the same or different and is an aromatic hydrocarbon group, preferably phenyl, and, R" and R"' represent a hydrogen atom, a carbonyl group, a carboxylic acid ester or carboxylic acid amide, a hydrocarbon group that contains up to 20 carbon atoms and may be substituted by one or more groups selected from carbonyl groups, carboxylic acid ester groups and carboxylic acid amide groups and/or which may form a ring are described in British Pat. No. 1,398,197. These catalysts are described as being more selective than previously proposed catalysts such as alkali or substances having alkaline properties i.e., amines, quaternary ammonium compounds, or a salt such as sodium acetate. Moreover, the phosphoranes are said to be more effective in promoting acceptable reaction rates than the latter group of catalysts.
The use of similar phorphorous-containing compounds, such as the alkylene phosphoranes and phosphonium halides, are also mentioned in German Offen. No. 2,643,336 (=CA, 87, 54629b (1977)) and British Pat. No. 1,485,345 as catalysts in the so-called "Taffy" process of preparing higher molecular weight solid resins directly from polyphenols and epichlorohydrin/caustic.